This datapack will allow you to turn all your slabs back to full blocks (2x slabs --> 1x Block). Just put one slab above the other and get back your blocks! If it's not possible due to already existing recipe (e.g. Stone Brick Slab->Chiseled Stone Bricks), put the two slabs anywhere else and it'll work.

You can also get back full blocks from all types of stairs(2x Stairs --> 1x Block OR 4x Stairs --> 3x Block), though you get only some amount of original materials used to produce the stairs. It's for getting something out of old stairs you don't need anymore.

You can choose if you want to use both or only one of them, e.g. if you only want to use the stairs portion of the datapack.

Installation:

1. Download this datapack
2. Put the zip file into your datapacks folder in "%appdata%\.minecraft\saves\(name_of_your_world)\datapacks"
3. If you don't want to use slabs or stairs, just delete the folder you don't want in the zip file in "Reverse Craft - Slabs & Stairs\data\reversecraft\recipes\"
4. Done

Let me know if you find out something is wrong or needs a change. Thanks and enjoy.